Parasitic fly for ID please

Observation - Parasitic fly for ID please - Global. Description: Whatever the fly did to this caterpillar it had an almost instant effect. Within a few seconds of the attack the caterpillar started writhing around and then fell off the nettle and I lost it. Thanks for looking.

Whatever the fly did to this caterpillar it had an almost instant effect. Within a few seconds of the attack the caterpillar started writhing around and then fell off the nettle and I lost it. Thanks for looking.